Question

What is the exact use of Utilities.sleep() function? Should we use it between function calls or API calls?

I use the Utilities.sleep(1000) in-between function calls, is it right? Will it slow down the execution time?

Answer 1

Utilities.sleep(milliseconds) creates a ‘pause’ in program execution, meaning it does nothing during the number of milliseconds you ask.
It surely slows down your whole process and you shouldn’t use it between function calls.
There are a few exceptions though, at least that one that I know : in SpreadsheetApp when you want to remove a number of sheets you can add a few hundreds of millisecs between each deletion to allow for normal script execution (but this is a workaround for a known issue with this specific method). I did have to use it also when creating many sheets in a spreadsheet to avoid the Browser needing to be ‘refreshed’ after execution.

Here is an example :

function delsheets(){
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var numbofsheet=ss.getNumSheets();// check how many sheets in the spreadsheet
  for (pa=numbofsheet-1;pa>0;--pa){ 
    ss.setActiveSheet(ss.getSheets()[pa]);
    var newSheet = ss.deleteActiveSheet(); // delete sheets begining with the last one
    Utilities.sleep(200);// pause in the loop for 200 milliseconds
  }
  ss.setActiveSheet(ss.getSheets()[0]);// return to first sheet as active sheet (useful in 'list' function)
}